Abstract
2.1. Introduction
In this chapter, I have mainly discussed the synthesis of new well-defined NNS- Mn(I) complexes and explored their applicability in acceptorless dehydrogenation and borrowing hydrogen catalysis. In view of sustainable and cost-effective development, I am particularly interested in the use of earth-abundant biocompatible manganese metal and non-phosphine ligand system. The syntheses of amines, imines and 2,3- dihydro‑1H‑perimidine were achieved using the catalytic efficacy of these manganese complexes.
Nitrogenous compounds such as amines and imines are known for their valuable application in chemistry.1 They are ubiquitous in many natural products and widely used as dyes, pharmaceuticals, agrochemicals, lubricants and surfactants.2 The two most widely used methods for the synthesis of amines are the Buchwald-Hartwig amination3 and Ullmann reactions.4 Although these methods are most significant in the organic synthesis, they often suffer from the generation of substantial amounts of side products or wastes. In recent year, much focus has been attributed to the conversion of alcohols to amines, as the alcohols are easily available either by different industrial processes or can be obtained renewably from lignocellulose5. The classical way to form C-N bond from alcohols is converting first the alcohol functionality to a suitable leaving group such as halides, triflates, tosylates, or mesylates and then reacting with primary amines to get the N-alkylated product.6 These multistep strategies use hazardous reagents and lengthy work-up procedures, which generates a large amount of waste.7 To overcome these drawbacks, new catalytic protocols involving direct N-alkylation of amines by alcohol using hydrogen autotransfer or borrowing hydrogen strategies have been developed.
The catalytic cycle involves three steps:
i) Dehydrogenation of an alcohol to form an aldehyde/ketone
ii) Imine formation via condensation of amine and the in situ formed
iii) Hydrogenation of the imine to form amine (hydrogen auto-transfer or borrowing hydrogen).
Scheme 2.1: Synthesis of amine and imine through borrowing hydrogen or acceptorless dehydrogenation.
Initially, the pioneering works on the N-alkylation of amines by alcohols were described independently by Watanabe8 and Grigg9 at the beginning of the 1980s, the
Scheme 2.2: Synthesis of amine catalysed by Ru/Os-pincer complexes.
significant progress has been achieved only after 2000 using precious noble metals.10-14 Some of the noble metal catalyzed N-alkylation reactions have been discussed here. In 2011, Gusev and co-workers12 reported various phosphine containing pincer-type complexes of Os and Ru and studied their catalytic activities toward N-alkylation of amines with alcohols. The reaction occurred at higher temperature (140-200 °C). They have noticed that the dihydride complexes of PNP[=HN(C2H4PiPr2)2] are more reactive than POP[=O(C2H4PiPr2)2] dihydride complexes (Scheme 2.2). In 2012, the group of Kempe demonstrated an iridium catalyzed11l synthesis of symmetrical and unsymmetrical monoalkylation of ortho-, meta- and para-benzenediamines (Scheme 2.3) via hydrogen auto-transfer reaction in presence of excess tBuOK (2.2 mmol).
Scheme 2.3: Synthesis of di-alkylated amine catalysed by Ir-pincer complex.
The replacement of precious noble metal such as ruthenium, rhodium, iridium, and osmium by eco-friendly, non-precious, earth-abundant 3d transition metals15 is a key challenge in the homogeneous catalysis. In recent years, explosive growth in the catalysis by base metals has been observed.
In 2013, Hanson and co-workers explored the applicability of cobalt(II) alkyl complex [(PNP)Co(CH2SiMe3)]BArF4 toward the dehydrogenative synthesis of imines16a directly from alcohol and amine. The reaction was performed in the presence of cobalt precatalyst 1.10a (1 mol%) in toluene at 120 °C. Zhang and co-workers16b have utilized the same cobalt complex for the efficient N-alkylation reaction of both aromatic and aliphatic amines with alcohol derivatives via borrowing hydrogen catalysis and they were able to isolate selectively amine products (74-96%). The reaction of
Scheme 2.4: Selective synthesis of imine and amine catalysed by Co-PNP complex.
primary amines and alcohols were performed in presence of cobalt precatalyst 1.10a (1 mol%) and 4 Å MS in toluene at 120 °C. In 2014, Feringa and Barta reported17 base free N-alkylation of amine in presence of a bifunctional air-stable Fe-precatalyst 1.12a and Me3NO. The N-alkylation reaction was performed in cyclopentyl methyl ether (CPME)solvent at 120 °C. A wide range of functional groups was well tolerated under the reaction condition. They have also reported the synthesis of different ring sized N- heterocycles by the reaction of amines with various diols (Scheme 2.5).
Scheme 2.5: N-alkylation of amine catalysed by Fe-PNP pincer complex.
The first example of nickel catalysed N-alkylation reaction followed by the borrowing hydrogen concept was reported in 1932 with aliphatic amines and primary alcohols.18
After that, the pioneering work of N-alkylation reaction was developed based on the heterogeneous nickel catalysts.19-21 Nickel catalysed homogenous N-alkylation of aryl amines and hydrazides were introduced by Zhou and co-workers22a in the year 2017.
They showed that 2 mol% Ni(OTf)2, 2.5 mol% dcpp [1,3-bis(dicyclohexylphosphino)- propane], and molecular sieves are essential for the reaction (Scheme 2.6, condition A).
Recently, Banerjee and co-workers also reported nickel catalysed monoalkylation of aniline22b with various primary alcohols. The reaction is catalyzed by 10 mol% NiBr2
with 20 mol% 1,10-phenanthroline in the presence of 100 mol% tBuOK as a base (Scheme 2.6, condition B). They have shown a wide range of functional group tolerance including hydroxyl, alkene, nitrile and nitro to establish the utility of their protocol.
Scheme 2.6: Synthesis of amine catalysed by Ni-complexes.
In the year 2016, manganese was first used towards the N-alkylation of alcohols with amine23a and the dehydrogenative synthesis of imines from alcohol and amine.23b In this year, Beller and coworker first demonstrated the efficient N-alkylation of alcohol
Scheme 2.7: Synthesis of amine catalysed by Mn-PNP pincer complexes.
using Mn-PNP Pincer complexes.23a Among these Mn-complexes, complex 1.16a was found to be the best catalyst for the amine synthesis in presence of 0.75 equivalent tBuOK at 80 °C temperature (Scheme 2.7). In the same year, Milstein and co-workers reported the first imine synthesis through the dehydrogenative coupling of alcohols and amines catalyzed by pyridine based Mn-PNP pincer complex (1.10c)23b in the presence of catalytic amount of tBuOK (3 mol%) (Scheme 2.8).
Scheme 2.8: Synthesis of imine catalysed by Mn-PNP pincer complexes.
Very recently, in 2018, Kempe and co-workers reported selective manganese- catalyzed23c synthesis of N-alkylated amines via borrowing hydrogen/hydrogen autotransfer and dehydrogenative imine synthesis. The reaction is catalysed by triazine based Mn-PNP pincer complex (Scheme 2.9).
Scheme 2.9: Selective amine and imine catalysed by Mn-PNP pincer complexes.
The literature reports emphasize that the methods for preparation amine and imines via borrowing hydrogen method/acceptorless dehydrogenation is highly important because of its usefulness. Hence, the development of new, selective, economically beneficial and environmentally benign catalytic protocols for these types
of processes is highly desirable. The reported manganese complexes are mainly derived from phosphine based ligand systems. Although phosphine ligands are recognized for the significant applications in the homogeneous catalysis, still, there are some drawbacks associated with their air and moisture sensitivities, complex synthetic procedures and relatively high cost. Herein, I am interested to study the coordination behavior of phosphine free SNS ligand systems with manganese precursor and investigate the applicability of the new metal complexes in acceptorless dehydrogenation and borrowing hydrogen catalysis.